Structure for improving static refresh

ABSTRACT

A double blanket ion implant method for forming diffusion regions in memory array devices, such as a MOSFET access device is disclosed. The method provides a semiconductor substrate with a gate structure formed on its surface. Next, a first pair of diffusion regions are formed in a region adjacent to the channel region by a first blanket ion implantation process. The first blanket ion implantation process has a first energy level and dose. The device is subjected to oxidizing conditions, which form oxidized sidewalls on the gate structure. A second blanket ion implantation process is conducted at the same location as the first ion implantation process adding additional dopant to the diffusion regions. The second blanket ion implantation process has a second energy level and dose. The resultant diffusion regions provide the device with improved static refresh performance over prior art devices. In addition, the first and second energy levels and doses are substantially lower than an energy level and dose used in a prior art single implantation process.

This application is a divisional of application Ser. No. 09/532,094,filed on Mar. 21, 2000, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor memorydevices and, more particularly to a structure having improved staticrefresh properties in dynamic random access memory devices and a methodof making it.

2. Description of the Related Art

Metal oxide semiconductor (MOS) structures are basic electronic devicesused in many integrated circuits. One such structure is the metal oxidesemiconductor field effect transistor (MOSFET), which is typicallyformed in a semiconductor substrate by providing a gate structure overthe substrate to define a channel region, and by forming source anddrain regions on opposing sides of the channel region.

To keep pace with the current trend toward maximizing the number ofcircuit devices contained in a single chip, integrated circuit designerscontinue to design integrated circuit devices with smaller and smallerfeature sizes. For example, not too long ago it was not uncommon to haveMOSFET devices (including CMOS devices) having channel lengths of 2microns or more. The current state of the art for production MOSFETdevices includes channel lengths of less than a ¼ micron.

As the channel lengths of MOSFET devices have been reduced, MOSFETS havebecome more susceptible to certain problems. One common problem isincreased junction leakage, a condition affecting the refreshcharacteristics of a dynamic random access memory (DRAM) memory cell.DRAM is a specific category of random access memory (RAM) containing anarray of individual memory cells, where each cell includes a capacitorfor holding a charge and a transistor for accessing the charge held inthe capacitor. Due to junction leakage, the stored charge must bere-stored in the capacitor on a periodic basis through a process knownas refresh. Increased junction leakage leads to a premature depletion ofthe capacitor's stored charge, necessitating more frequent refreshcycles. Because resources are expended in refreshing the DRAM cells, thelonger the period between refresh cycles, the better. The term “pause”is often used to represent the amount of time that a DRAM cell, or groupof cells, can maintain their charge without undergoing a refreshoperation. That is, how long can the DRAM control circuitry pausebetween refresh operations and still maintain the stored state of theDRAM memory cell. It is desirable to extend the pause period of, andimprove the static refresh of, the DRAM.

A manufacturer may want to improve static refresh performance of theDRAM to provide customers with the capability to perform more memoryoperations (e.g., reads and writes) between refresh cycles. This reducesthe overhead required to utilize the DRAM. Moreover, a manufacturer maywant to improve static refresh performance to improve the operatingspecifications of the DRAM. For example, DRAMs typically have alow-power or standby specification requiring the DRAM to operate withina maximum current during a low-power mode. Since memory cells must berefreshed during the lower-power mode, reducing the frequency of therefresh operations will improve the DRAM's operational performance forthe low-power mode.

FIG. 1 illustrates a prior art MOSFET memory array device 5. The device5 and its fabrication method are described in U.S. Pat. No. 5,534,449(Dennison et al.), which is hereby incorporated by reference in itsentirety. Briefly, the fabrication of the device 5 is initiated byforming a gate structure 10 on a substrate 8. The substrate 8 istypically a bulk silicon substrate, which may have a doped well thereinin which transistors are formed. The gate structure 10 (referred to inthe ′449 patent as a gate line) typically comprises a gate oxide 12, aconductive polysilicon layer 14, an overlying WSi_(x) layer 16, anoverlying novellus oxide layer 18 and a Si₃N₄ capping layer 20. Thecross sectional width of this prior art gate structure 10 is 0.40microns.

Once the gate structure 10 is formed, the device 5 is subjected tooxidizing conditions. This process step is often referred to as a“re-ox” step or a thermal re-ox step. Oxidized sidewalls 22, 24 areformed on the gate structure 10, and oxide regions 26, 28 are formed onthe substrate, as a result of the re-ox step. Subsequent to the re-oxstep, a blanket phosphorous implant step is performed to form diffusionregions 30, 32. This blanket phosphorous implant is performed at anenergy level ranging from 30 Kev to 60 Kev with a dose ranging from7×10¹² ions/cm² to 1.5×10¹³ ions/cm² to provide an average dopantconcentration for the diffusion regions 30, 32 ranging from 1×10¹⁷ions/cm³ to 1×10¹⁹ ions/cm³. For the prior art device 5, this blanketphosphorous implant step is performed after the re-ox step to preventthe phosphorous from diffusing too far underneath the gate structure 10,which could cause transistor leakage problems.

The fabrication process of the device 5 typically includes the formationof oxide or nitride sidewall spacers 40, 42 on the sidewalls of the gatestructure 10. Further processing may be performed as described in the'449 patent. Although the MOSFET memory array device 5 is a vastimprovement over earlier memory array devices, it can still benefit fromimproved static refresh performance. Thus, it is still desirable toimprove as much as possible the static refresh performance of the memorydevice.

SUMMARY OF THE INVENTION

The present invention provides a memory array device having improvedstatic refresh over prior art memory array devices.

The above and other features and advantages of the invention areachieved by a double blanket ion implant method for forming diffusionregions in memory array devices, such as a MOSFET access device. Themethod provides a semiconductor substrate with a gate structure formedon its surface. Next, a first pair of diffusion regions are formed in aregion adjacent to the channel region by a first blanket ionimplantation process. The first blanket ion implantation process has afirst energy level and dose. The device is subjected to oxidizingconditions, which form oxidized sidewalls on the gate structure. Asecond blanket ion implantation process is conducted at the samelocation as the first ion implantation process adding additional dopantto the diffusion regions. The second blanket ion implantation processhas a second energy level and dose. The resultant diffusion regionsprovide the device with improved static refresh performance over priorart devices. In addition, the first and second energy levels and dosesare substantially lower than an energy level and dose used in a priorart single implantation process.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of the preferredembodiments of the invention given below with reference to theaccompanying drawings in which:

FIG. 1 is a fragmentary vertical cross-sectional view of a prior artmemory array device conventional diffision regions;

FIG. 2 is a fragmentary vertical cross sectional view of an integratedcircuit memory array device formed in accordance with the presentinvention;

FIG. 3 is a fragmentary vertical cross sectional view of the deviceillustrated in FIG. 2 at an early stage of formation;

FIG. 4 is a fragmentary vertical cross sectional view of the deviceillustrated in FIG. 3 at a later stage of formation;

FIG. 5 is a fragmentary vertical cross sectional view of the deviceillustrated in FIG. 4 at a later stage of formation;

FIG. 6 is a fragmentary vertical cross sectional view of the deviceillustrated in FIG. 5 at a later stage of formation;

FIG. 7 is a fragmentary vertical cross sectional view of the deviceillustrated in FIG. 6 at a later stage of formation;

FIG. 8 is a graph illustrating the dopant concentration of diffusionregions within the devices illustrated in FIGS. 1 and 2;

FIGS. 9 and 10 are graphs illustrating the static refresh performance ofthe devices illustrated in FIGS. 1 and 2; and

FIG. 11 is block diagram of a processor-based system including a memorydevice formed in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described as set forth in the preferredembodiments illustrated in FIGS. 2-7 and 11. Other embodiments may beutilized and structural or logical changes may be made without departingfrom the spirit or scope of the present invention. Like items arereferred to by like reference numerals.

FIG. 2 illustrates a portion of an integrated circuit MOSFET memoryarray device 105 constructed in accordance with the present invention.The device 105 is preferably used as an access device of a DRAM memorycell. As will be described with reference to FIGS. 3 to 7, the device105 including diffusion regions 130, 132 is fabricated using two blanketphosphorous ion implant steps sandwiched around a conventional re-oxstep. Since two implant steps are performed, diffusion region 130comprises two regions 130 a, 130 b having different dopantconcentrations. Similarly, diffusion region 132 comprises two regions132 a, 132 b having different dopant concentrations. As described withreference to FIGS. 9 and 10, the uniquely formed diffusion regions 130,132 provide the device 105 with improved static refresh performance overthe prior device 5 (illustrated in FIG. 1). Since the method uses twoseparate blanket phosphorous ion implant steps, it will be referred tohereinafter as a “double blanket ion implant method”.

Hereinafter, the terms “wafer” and “substrate” are used interchangeablyand are to be understood as including silicon-on-insulator (SOI) orsilicon-on-sapphire (SOS) technology, doped and undoped semiconductors,epitaxial layers of silicon supported by a base semiconductorfoundation, and other semiconductor structures. Furthermore, whenreference is made to a “wafer” or “substrate” in the followingdescription, previous process steps may have been utilized to formregions or junctions in the base semiconductor structure or foundation.

In addition, no particular order is required for the method stepsdescribed below, with the exception of those logically requiring theresults of prior steps, for example formation of spacers 40, 42 adjacentto the sidewalls of the gate structure 10 logically requires the priorformation of the gate structure 10 and its sidewalls. Otherwise,enumerated steps are provided below in an exemplary order which may bealtered, for instance the several ion implant steps may be rearrangedusing masking and etching steps as is known in the art.

FIG. 3 shows the integrated circuit MOSFET memory array device 105 inaccordance with the present invention at an early stage of formation. Agate structure 100 is provided on the substrate 8 as is known in the artand described in the '449 patent to Dennison et al. The substrate 8 istypically a bulk silicon substrate, which may have a doped well in whichaccess transistors are to be formed. The gate structure 110 comprises agate oxide 12, a conductive polysilicon layer 14, an overlying WSi_(x)layer 16, an overlying oxide layer 18 and a Si₃N₄ capping layer 20.Unlike the gate structure 10 of the prior art device 5 illustrated inFIG. 1, the cross sectional length of the gate structure 110 may besubstantially reduced. For example, the cross sectional length of thegate structure 110 can be substantially to approximately 0.20 microns.An advantage of the present invention is that the length of the gatestructure 110 is reduced in comparison to the prior art due to theunique fabrication processing of the present invention (describedbelow).

Referring now to FIG. 4, diffusion regions 130 a, 132 a are formed inthe substrate 8 adjacent the sidewalls of the gate structure 110 andextend laterally away from the gate structure 110. It should be notedthat a portion of the diffusion regions 130 a, 132 a diffuse beneath thegate structure 110. To create the diffusion regions 130 a, 132 a, thesubstrate 8 undergoes a first blanket implant step. It is desirable thatan n-type be used, which makes the device 105 an NMOS device. It isdesirable that the n-type dopant be phosphorous. However, it should benoted that other dopants can be used if so desired. For example, othern-type dopants such as arsenic or antimony could be used. If it weredesirable for the device 105 to be a PMOS device, a p-type dopant suchas boron, boron bifluoride (BF₂) or borane (B₂H₁₀,) could be used. Thisfirst blanket phosphorous implant may be performed, for example, at anenergy level of approximately 15 Kev with a dose of approximately 2×10¹²ions/cm². It should be appreciated that any other suitable dose andenergy level can be used for this step. One exemplary range for thefirst blanket phosphorous implant may include an energy level betweenapproximately 5 Kev to 45 Kev with a dose of approximately 1×10¹²ions/cm² to slightly less than 7×10¹² ions/cm².

It must be noted that this blanket phosphorous implant step is performedprior to a subsequent re-ox step since the energy level and dose issubstantially lower than the dose used in the prior art (i.e., energylevel ranging from 30 Kev to 60 Kev with a dose ranging from 7×10¹²ions/cm² to 1.5×10¹³ ions/cm² to provide an average dopant concentrationfor the diffusion regions 30, 32 ranging from 1×10¹⁷ ions/cm³ to 1×10¹⁹ions/cm³). Thus, the first blanket phosphorous implant step can beperformed prior to the re-ox step without having the phosphorous diffusetoo far underneath the gate structure 110 and without causing subsequenttransistor leakage problems.

Referring now to FIG. 5, a re-ox step is performed forming oxidizedsidewalls 22, 24 on the gate structure 110 and oxide regions 26, 28 onthe substrate 8. It should be appreciated that any conventional re-oxprocess can be performed at this point, such as a thermal re-ox processor a source/drain thermal re-ox process. Referring to FIG. 6, diffusionregions 130 b, 132 b are formed in the substrate 8 at the same locationas diffusion regions 130 a, 132 b. To create the second diffusionregions 130 b, 132 b, the substrate 8 undergoes a second blanket implantstep. As with the first blanket implant step, it is desirable that thedopant used is phosphorous. However, it should be noted that otherdopants can be used if so desired, particularly if a differentconductivity type of the device 105 is desired. This second blanketphosphorous implant may be performed at an energy level of approximately20 Kev with a dose of approximately 4×10¹² ions/cm². It should beappreciated that any other suitable dose and energy level can be usedfor this step. One exemplary range for the second blanket phosphorousimplant may include an energy level between approximately 5 Kev to 60Kev with a dose of approximately 1×10¹² ions/cm² to 1×10¹³ ions/cm².

The oxidized sidewalls 22, 24 on the gate structure 110 prevent thesecond implant from diffusing underneath the gate structure 110, whichhelps in the formation of the individual diffusion regions 130 a, 130 b,132 a, 132 b. The two diffusion regions 130 a, 130 b combine to form onediffusion region 130. The resultant diffusion region 130 will have twodifferent dopant concentrations, one from region 130 a and one fromregion 130 b. There will be a smooth transition between the dopantconcentrations of the two regions 130 a, 130 b. Similarly, the twodiffusion regions 132 a, 132 b combine to form one diffusion region 132.The resultant diffusion region 132 will have two different dopantconcentrations, one from region 132 a and one from region 132 b. Therewill be a smooth transition between the dopant concentrations of the tworegions 132 a, 132 b. As will be discussed below, these uniquely formeddiffusion regions 130, 132 allow the device 105 to have substantiallybetter static refresh performance in comparison to the prior art device5 (FIG. 1).

Referring to FIG. 7, oxide or nitride sidewall spacers 40, 42 may beformed on the on the sidewalls of the gate structure 110 (as describedin the '449 patent or by any other known method). In addition, furtherprocessing may be performed to form a memory cell as described in the'449 patent. It can be seen that the device 105 has two diffusionregions 130, 132, each having a pair of diffusion regions 130 a, 130 b,132 a, 132 b, respectively.

FIG. 8 illustrates an exemplary phosphorous concentration 150 of thesecond diffusion region 132 with respect to its length (illustrated byarrow X). It should be noted that the first diffusion region 130 wouldhave a similar concentration, but in a direction opposite the directionindicated by arrow X. An exemplary phosphorous concentration 152 of theprior art device is also illustrated. From the curves 150, 152 it can beseen how the second diffusion region 132 has a more graded concentrationof phosphorous than the prior art diffusion regions (e.g., region 32 inFIG. 1). By more graded, we mean that the net doping concentrationversus distance changes gradually. By contrast, as shown by curve 152,the diffusion region 32 (FIG. 1) of the prior art device has an abruptchange in concentration of phosphorous versus distance. That is, the netdoping concentration of the prior art curve 152 undergoes a steep changewith respect to distance. With a graded dopant concentration of thediffusion regions, the resistance to current flow is less than thediffusion regions of the prior art. Although the invention is not to bebound to any specific theory, it is believed that the more gradedconcentration of the present invention improves the static refresh ofthe device 105 by improving the junction at the storage node of the DRAMmemory cell.

Referring again to FIG. 7, it can be seen that the two diffusion regions130, 132 slightly diffuse below the gate structure 110. That is, thereis a first region 140 of the first diffusion region 130 that residesunderneath a portion of the gate structure 110. Similarly, there is asecond region 142 of the second diffusion region 132 that residesunderneath a portion of the gate structure 110. These regions 140, 142,which can be referred to as “overlap” regions, make the device 105 morerobust to reliability stressing. That is, the overlap regions 140, 142are less likely to degrade when high voltage is applied to the device,such as the types of voltages applied during manufacturing stresstesting. These regions 140, 142, which are not present in the prior artdevice 5 (FIG. 1), are formed by the first blanket phosphorous implantstep (FIG. 4). That is, by having the first blanket phosphorous implantstep (FIG. 4) prior to the re-ox step (FIG. 5) some dopant can diffuseunderneath the gate structure 110 forming region 140, 142 and causingthe device 105 to have the above-mentioned robustness. This is anotherbenefit of the present invention.

A standard measure of refresh performance is known as a “time tounrepairable calculation”. The term “repair” is sometimes used toindicate that a memory cell or memory bit has been repaired byelectrical replacement with a redundant element. The terms “un repaired”or “un-repairable” are often used to indicate that the number of failingbits exceeds the capability of repair by redundant elements. In the timeto un-repairable test, data is written into the bits of memory cells inthe DRAM array. Measurements are taken to determine when a predeterminednumber of bits have lost their charge and within what time. The time ittakes for the bits to lose their charge is commonly referred to as the“time to un-repairable” (TTUR).

Referring now to FIGS. 1, 2 and 9. The inventors ran experiments tocompare TTUR results using the prior art device 5 (FIG. 1) with theresults using the device 105 (FIG. 2) constructed in accordance with thepresent invention. FIG. 9 illustrates results from TTUR tests based onfinding 100 bits that have lost their charge. The y-axis indicates theprobability that 100 bits have lost their charge. The x-axis indicatesthe time when the charge was lost (and when a refresh operation becamenecessary). The first set of data 160 illustrates the results using thedevice 105 of the present invention. The second set of data 162illustrates the results using the device 5 of the prior art. From thedata 160, 162, it can be seen that 100 bits lost their charge (with 50%probability, i.e., 0.5 on the y-axis) using the prior art device 5 atapproximately 120 milliseconds, while 100 bits lost their charge usingthe device 105 at approximately 210 milliseconds. That is, there isalmost a 90 millisecond improvement in the device 105 constructed inaccordance with the present invention. It is believed that thisimprovement is due to the uniquely formed diffusion regions 130, 132 ofthe device 105.

Referring now to FIGS. 1, 2 and 10. FIG. 10 illustrates results fromTTUR tests based on finding 200 bits that have lost their charge. They-axis indicates the probability that 200 bits have lost their charge.The x-axis indicates the time when the charge was lost (and when arefresh operation became necessary). The first set of data 170illustrates the results using the device 105 while the second set ofdata 172 illustrates the results using the device 5. From the data 170,172, it can be seen that 200 bits lost their charge (with 50%probability, i.e., 0.5 on the y-axis) using the prior art device atapproximately 240 milliseconds, while 200 bits lost their charge usingthe device 105 at approximately 310 milliseconds. That is, there isalmost a 70 millisecond improvement.

FIG. 11 illustrates a block diagram of a processor based system 200utilizing a DRAM memory circuit 208 constructed in accordance with thepresent invention. That is, the memory circuit 208 utilizes the MOSFETmemory array device 105 (FIG. 2) constructed in accordance with thepresent invention (FIGS. 3 to 7). The processor-based system 200 may bea computer system, a process control system or any other systememploying a processor and associated memory. The system 200 includes acentral processing unit (CPU) 202, e.g., a microprocessor, thatcommunicates with the DRAM memory circuit 208 and an I/O device 204 overa bus 220. It must be noted that the bus 220 may be a series of busesand bridges commonly used in a processor-based system, but forconvenience purposes only, the bus 220 has been illustrated as a singlebus. A second I/O device 206 is illustrated, but is not necessary topractice the invention. The processor-based system 200 also includes aread-only memory (ROM) circuit 210 and may include peripheral devicessuch as a floppy disk drive 212 and a compact disk (CD) ROM drive 214that also communicates with the CPU 202 over the bus 220 as is wellknown in the art. It should be noted that the CPU 202 can be combined ona single chip with one or more DRAM memory circuits 208 and ROM circuits210.

While the invention has been described in detail in connection with thepreferred embodiments known at the time, it should be readily understoodthat the invention is not limited to such disclosed embodiments. Rather,the invention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Accordingly, the invention is not to be seen as limited bythe foregoing description, but is only limited by the scope of theappended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. An integrated circuit semiconductor devicecomprising: a substrate having a first surface; a gate structure formedon said first surface; and first and second diffusion regions formedwithin said substrate on opposite sides of said gate structure, whereineach of said diffusion regions comprise first and second portionsrespectively having first and second dopant concentrations, which aredifferent and cause each diffusion region to have a graded dopantconcentration; wherein said first diffusion region is partially locatedunderneath a thermally re-oxidized first side wall of said gatestructure and said second diffusion region is partially locatedunderneath a thermally re-oxidized second side wall of said gatestructure.
 2. The device of claim 1 wherein said first and seconddiffusion regions are doped with phosphorous.
 3. A processor-basedsystem comprising: a processor, and an integrated circuit semiconductordevice coupled to said processor, said integrated circuit semiconductordevice comprising: a substrate having a first surface; a gate structureformed on said first surface; and first and second diffusion regionsformed within said substrate on opposite sides of said gate structure,wherein each of said diffusion regions comprise first and secondportions respectively having first and second dopant concentrations,which are different and cause each diffusion region to have a gradeddopant concentration.
 4. A processor-based system comprising: aprocessor; and an integrated circuit semiconductor device coupled tosaid processor, said integrated circuit semiconductor device beingintegrated on a same chip as said processor, said integrated circuitsemiconductor device comprising: a substrate having a first surface; agate structure formed on said first surface; and first and seconddiffusion regions formed within said substrate on opposite sides of saidgate structure, wherein each of said diffusion regions comprise firstand second portions respectively having first and second dopantconcentrations, which are different and cause each diffusion region tohave a graded dopant concentration.